linux-kernel - Re: Kernel stack read with PTRACE_EVENT_EXIT and io

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Date:   Mon, 21 Jun 2021 19:45:38 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Michael Schmitz <schmitzmic@...il.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        alpha <linux-alpha@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Ley Foon Tan <ley.foon.tan@...el.com>,
        Tejun Heo <tj@...nel.org>, Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

On Mon, Jun 21, 2021 at 12:22:06PM -0700, Linus Torvalds wrote:
> On Mon, Jun 21, 2021 at 11:59 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> >         There's a large mess around do_exit() - we have a bunch of
> > callers all over arch/*; if nothing else, I very much doubt that really
> > want to let tracer play with a thread in the middle of die_if_kernel()
> > or similar.
> 
> Right you are.
> 
> I'm really beginning to hate ptrace_{event,notify}() and those
> PTRACE_EVENT_xyz things.
> 
> I don't even know what uses them, honestly. How very annoying.
> 
> I guess it's easy enough (famous last words) to move the
> ptrace_event() call out of do_exit() and into the actual
> exit/exit_group system calls, and the signal handling path. The paths
> that actually have proper pt_regs.
> 
> Looks like sys_exit() and do_group_exit() would be the two places to
> do it (do_group_exit() would handle the signal case and
> sys_group_exit()).

Maybe...  I'm digging through that pile right now, will follow up when
I get a reasonably complete picture.  In the meanwhile, do kernel/kthread.c
uses look even remotely sane?  Intentional - sure, but it really looks
wrong to use thread exit code as communication channel there...